disk: remove unexisting local variable 'resource'

this fixes a bug introduced in commit:
fb62cdc iaas: remove possible duplicates when passing multiples resources

Signed-off-by: William Dauchy <william@gandi.net>
base: fix error message for step unknown

when getting error unknown, it's not necessary a creation operation
this patch proposes to improve the output

Signed-off-by: William Dauchy <william@gandi.net>